System for providing keys

ABSTRACT

A method for providing keys for descrambling scrambled content, the scrambled content being divided into frames, the method including: identifying a plurality of frames to be descrambled; for each frame of the plurality of frames, identifying a key period associated with a key for descrambling the frame; and for at least one frame f of the plurality of frames, substituting a substitute frame g for the frame f, the substitute frame g being chosen in order to reduce a total number of key periods associated with keys for descrambling all of the plurality of frames. Related apparatus and methods are also provided.

The present application claims the benefit of the filing date ofprovisional application Ser. No. 60/514,659 filed on 27 Oct. 2003.

FIELD OF THE INVENTION

The present invention relates to conditional access in general, andparticularly but not exclusively to conditional access systems for usewith personal video recorders.

BACKGROUND OF THE INVENTION

Systems for scrambling a television data stream are well-known in theart. One such system is described in the following U.S. Pat. Nos.:5,282,249 to Cohen et al.; 5,481,609 to Cohen et al. Scrambledtelevision data streams described in the Cohen et al patents compriseboth scrambled data representing television signals and coded controlmessages, also known as ECMs. The ECMs of Cohen et al comprise, in acoded form, data necessary for generating a control word (CW) which maybe used to descramble the scrambled data representing televisionsignals.

Data necessary for generating a control word may comprise all or part ofan ECM. In the present specification and claims, for sake of simplicityof description, generation of a control word is sometimes described astaking place based on an ECM, it being appreciated that all or part ofan ECM may be used. Data necessary for generating a control word isknown in the art to take many different forms and may include, ingeneral, at least any of the following: a control word; an encryptedcontrol word packet which is intended to be decrypted before use; and aseed to a generating function such as, for example, a one-way functionwhich generates the control word upon input of the seed. Control wordsare also termed “keys”; the terms “control word” and “key”, in all oftheir grammatical forms, are used interchangeably throughout the presentspecification and claims. Throughout the present specification andclaims the terms “control word generating information” and “CWgenerating information” are used interchangeably to designate datanecessary for generating a control word in any appropriate form, asdescribed above.

While the two patents to Cohen et al describe an analog system, that is,a system in which analog television data streams are broadcast totelevision sets, it is appreciated that similar ECM methods may also beused for digital television data streams. Generally, the scramblingtechniques used for scrambling analog television signals such as, forexample, the well-known “cut-and-rotate” technique, are chosen for theirapplicability to analog signals. In scrambling of digital televisionsignals other scrambling techniques, well-known in the art, are used,the techniques being more appropriate to digital signals such as, forexample, applying the well-known DES algorithm, the well-knowntriple-DES algorithm, or the DVB Common Scrambling Algorithm to thedigital television signals; it is appreciated that any appropriatescrambling technique may be used.

Methods of transmitting a scrambled digital signal, including ECMs, aredescribed in the MPEG-2 standard, ISO/IEC 13818-6, 12 Jul. 1996 andsubsequent editions. In general, methods of transmitting a scrambleddigital signal, including ECMs, are well known. Some systems use acomposite signal, that is, an analog or digital signal including aplurality of components. Typically the plurality of components includesboth a scrambled television signal component and an ECM component, theECM component comprising a plurality of ECMs. Prior art references whichdiscuss examples of this type of signal include the following: DVBETR289, “Digital Video Broadcasting (DVB): Support for use of scramblingand conditional access (CA) within digital broadcasting systems”; andDVB SIM061, “Technical specification of DVB-Simulcrypt”, 1 Apr. 1997.

The system of Cohen et al and other systems described in the art share afeature of broadcasting an ECM synchronous to, generally immediatelybefore or in a data stream parallel to, the actual scrambled broadcast.Typically, the CW or key which can be generated from such an ECM (orfrom CW generating information comprised therein) is valid for a briefduration known as a “key period”. Typically, a key period lasts for afew seconds, such as 8 seconds or 10 seconds. Each key period isassociated with a segment of scrambled data or content which can bedescrambled using the key associated with that key period. The conceptof key period is explained in the above-mentioned DVB ETR289 reference,as well as in Simulcrypt specification ETSI TS 101 197 and Simulcryptguidelines ETSI TR 102 035.

It is well known in broadcasting to provide many ECMs for each keyperiod so that, for example, if one tunes to a new broadcast program oneneed not wait for the next key period to begin in order to view thebroadcast; rather, one need only wait for the next ECM. Typically, anECM is broadcast a plurality of times, such as 10 or 20 times, per keyperiod.

When scrambled data is recorded for later descrambling and playback, itis typically only necessary to store one ECM per key period. One exampleof a system for recording and playing back scrambled data is describedin U.S. Pat. No. 6,178,242 to Tsuria, the disclosure of which is herebyincorporated herein by reference.

Another example of a system for recording and playing back scrambleddata is described in the following patents and patent applications, thedisclosures of which are hereby incorporated herein by reference: PCTPublished Patent Application WO 00/01149 of NDS Limited; correspondingEuropean Patent 1 013 088 of NDS Limited; and U.S. patent applicationSer. No. 09/515,118 of Wachtfogel et al, assigned to NDS Limited. Such asystem is one example of a type of system known in the art as a PersonalVideo Recorder or PVR.

Playing back a scrambled recorded data stream presents certainchallenges, some of which have been addressed in the following patentsand published patent applications, the disclosures of which are herebyincorporated herein by reference:

1. PCT Published Patent Application WO 01/35669 of NDS Limited;corresponding European Patent 1 142 340 of NDS Limited; andcorresponding U.S. patent application Ser. No. 09/574,096 of Darshan etal, assigned to NDS Limited; and

2. PCT Published Patent Application WO 03/010970 of NDS Limited.

The disclosures of all references mentioned above and throughout thepresent specification, as well as the disclosures of all referencesmentioned in those references, are hereby incorporated herein byreference.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved conditional accesssystem, particularly but not exclusively for use with personal videorecorders (PVRs).

The inventors of the present invention believe that a problem may existwhen playback of recorded scrambled content, particularly but notexclusively video content, is attempted at high “fast forward” or “fastreverse” speed. Persons skilled in the art will appreciate, in light ofthe above discussion, that, in normal speed playback, a key fordescrambling the scrambled content must be produced for every keyperiod. In fast forward playback, only 1 in every n frames of contentmust be displayed; however, if n is less than the number of frames in akey period, it is still necessary to produce a key for descrambling thescrambled content for every key period.

When playback is very fast, the necessity of producing keys from ECMsmay strain the abilities of the playback system. This is because, inplayback of a “live” broadcast stream, only one ECM must be processed toproduce a key for each key period; in fast forward playback, bycontrast, up to one ECM must be processed for each frame. It willtherefore be appreciated that a large number of ECMs must be processedeach second during fast forward playback.

In preferred embodiments of the present invention, a substitute frame ischosen for at least one frame which is to be descrambled, in such a wayas to reduce the number of different key periods for which a frame is tobe descrambled. Thus, the number of ECMs which must be processed toproduce keys is also reduced, and the above-described problem isovercome.

The term “render” is used, in all its grammatical forms, throughout thepresent specification and claims to refer to any appropriate mechanismor method of making content palpable to one or more of the senses. Inparticular and without limiting the generality of the foregoing,“render” refers not only to display of video content but also toplayback of audio content.

There is thus provided in accordance with a preferred embodiment of thepresent invention a method for providing keys for descrambling scrambledcontent, the scrambled content being divided into frames, the methodincluding identifying a plurality of frames to be descrambled, for eachframe of the plurality of frames, identifying a key period associatedwith a key for descrambling the frame, and for at least one frame f ofthe plurality of frames, substituting a substitute frame g for the framef, the substitute frame g being chosen in order to reduce a total numberof key periods associated with keys for descrambling all of theplurality of frames.

Further in accordance with a preferred embodiment of the presentinvention the method also includes producing one key for each keyperiod.

Still further in accordance with a preferred embodiment of the presentinvention the method also includes producing exactly one key for eachkey period.

Additionally in accordance with a preferred embodiment of the presentinvention the producing includes producing from an ECM.

Moreover in accordance with a preferred embodiment of the presentinvention the method also includes rendering at least one of theplurality of frames.

Further in accordance with a preferred embodiment of the presentinvention the rendering includes rendering all of the plurality offrames.

Still further in accordance with a preferred embodiment of the presentinvention the substituting includes determining a number NK of keyperiods to be skipped for each key period to be descrambled, NK beinggreater than or equal to 1, determining a plurality of groups of keyperiods, each group including one key period to be descrambled and NKkey periods to be skipped, determining a number N of frames to beprovided from each group of the plurality of groups of key periods, andfor each one group of the plurality of groups of key periods, choosing Nframes to be provided from within the one key period to be descrambled,which one key period is included in the one group.

Additionally in accordance with a preferred embodiment of the presentinvention N is greater than or equal to 2.

Moreover in accordance with a preferred embodiment of the presentinvention the N frames are evenly spaced within the one key period.

There is also provided in accordance with another preferred embodimentof the present invention a method for providing keys for descramblingscrambled content, the scrambled content being divided into frames, themethod including identifying a plurality of frames to be descrambled,for each frame of the plurality of frames, identifying a key periodassociated with a key for descrambling the frame, and for each frame fof the plurality of frames, determining whether to substitute asubstitute frame g for the frame f based, at least in part, on adistance between frame g and frame f, and, at least in part, on a goalof reducing a total number of key periods associated with keys fordescrambling all of the plurality of frames, and based on a result ofthe determining, substituting frame g for frame f.

Further in accordance with a preferred embodiment of the presentinvention the method also includes producing one key for each keyperiod.

Still further in accordance with a preferred embodiment of the presentinvention the method also includes producing exactly one key for eachkey period.

Additionally in accordance with a preferred embodiment of the presentinvention the producing includes producing from an ECM.

Moreover in accordance with a preferred embodiment of the presentinvention the method also includes rendering at least one of theplurality of frames.

Further in accordance with a preferred embodiment of the presentinvention the rendering includes rendering all of the plurality offrames.

There is also provided in accordance with another preferred embodimentof the present invention apparatus for providing keys for descramblingscrambled content, the scrambled content being divided into frames, theapparatus including a frame identifier identifying a plurality of framesto be descrambled, a key period identifier operative, for each frame ofthe plurality of frames, to identify a key period associated with a keyfor descrambling the frame, and a frame substitutor operative, for atleast one frame f of the plurality of frames, to substitute a substituteframe g for the frame f, the substitute frame g being chosen in order toreduce a total number of key periods associated with keys fordescrambling all of the plurality of frames.

There is also provided in accordance with yet another preferredembodiment of the present invention apparatus for providing keys fordescrambling scrambled content, the scrambled content being divided intoframes, the apparatus including a frame identifier identifying aplurality of frames to be descrambled, a key period identifieroperative, for each frame of the plurality of frames, to identify a keyperiod associated with a key for descrambling the frame, and a framesubstitutor operative, for each frame f of the plurality of frames, todetermine whether to substitute a substitute frame g for the frame fbased, at least in part, on a distance between frame g and frame f, and,at least in part, on a goal of reducing a total number of key periodsassociated with keys for descrambling all of the plurality of frames,and, based on a result of the determining, to substitute frame g forframe f.

There is also provided in accordance with still another preferredembodiment of the present invention apparatus for providing keys fordescrambling scrambled content, the scrambled content being divided intoframes, the apparatus including means for identifying a plurality offrames to be descrambled, means for identifying, for each frame of theplurality of frames, a key period associated with a key for descramblingthe frame, and means for substituting, for at least one frame f of theplurality of frames, a substitute frame g for the frame f, thesubstitute frame g being chosen in order to reduce a total number of keyperiods associated with keys for descrambling all of the plurality offrames.

There is also provided in accordance with yet another preferredembodiment of the present invention apparatus for providing keys fordescrambling scrambled content, the scrambled content being divided intoframes, the apparatus including means for identifying a plurality offrames to be descrambled, means for identifying, for each frame of theplurality of frames, a key period associated with a key for descramblingthe frame, and means for performing the following for each frame f ofthe plurality of frames: determining whether to substitute a substituteframe g for the frame f based, at least in part, on a distance betweenframe g and frame f, and, at least in part, on a goal of reducing atotal number of key periods associated with keys for descrambling all ofthe plurality of frames; and based on a result of the determining,substituting frame g for frame f.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 is a simplified partly pictorial, partly block diagramillustration of a PVR-based television apparatus including a system forproviding keys for descrambling content, constructed and operative inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a simplified pictorial illustration of scrambled framescomprised in a plurality of key periods, useful for understanding thesystem of FIG. 1;

FIG. 3 is a simplified pictorial illustration of scrambled framescomprised in a plurality of key periods, showing substitution of framesto be descrambled in order to reduce a number of key periods for which aframe is to be descrambled, useful for understanding the system of FIG.1;

FIG. 4 is a simplified block diagram illustration of a preferredimplementation of a portion of the system of FIG. 1;

FIG. 5 is a simplified flowchart illustration of a preferred method ofoperation of the system of FIG. 1; and

FIG. 6 is a simplified flowchart illustration of a preferredimplementation of a portion of the method of FIG. 5.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference is now made to FIG. 1, which is a simplified partly pictorial,partly block diagram illustration of a PVR-based television apparatusincluding a system for providing keys for descrambling content,constructed and operative in accordance with a preferred embodiment ofthe present invention. In the system of FIG. 1, generally designated 10,a television 15 is shown displaying a scene provided for rendering by apersonal video recorder (PVR) 20.

The PVR 20 is preferably implemented in any suitable combination ofsoftware and hardware, as is well known in the art. The PVR 20preferably comprises suitable conventional components (not shown), asare well known in the art. Non-limiting examples of conventional PVRsystems well known in the art include:

-   -   the system described in PCT Published Patent Application WO        00/01149 of NDS Limited, corresponding European Patent 1 013 088        of NDS Limited, and U.S. patent application Ser. No. 09/515,118        of Wachtfogel et al, referred to above; and    -   the XTV™ system, commercially available from NDS Limited, One        London Road, Staines, Middlesex TW18 4EX, United Kingdom.

In addition to conventional components, the PVR 20 preferably comprisesa trick modes unit 30, in operative association with a frame provisionsystem 40. The frame provision system 40 preferably comprises a framesubstitutor 50.

It is appreciated that various sub-combinations of the apparatus of FIG.1 may comprise alternative preferred embodiments of the presentinvention. For example, and without limiting the generality of theforegoing, each of the following may comprise an alternative preferredembodiment of the present invention:

-   -   1. the frame provision system 40; and    -   2. the frame substitutor 50.

The operation of the apparatus of FIG. 1 is now briefly described.

A user (not shown) requests a “trick mode” such as, for example, fastforward or fast reverse rendering of content stored in the PRV 20. Forpurposes of simplicity of description, fast forward, fast reverse, andother trick modes are sometimes termed herein “fast forward”; personsskilled in the art will appreciate that the invention is not therebylimited.

In order to fulfill the request, the trick modes unit 30 sends a request(not shown in FIG. 1) to provide a list of frames to be displayed to theframe provision system 40.

In an environment such as a home network environment, the requestpreferably comprises a request from a client to a server to play from agiven place at a given speed and direction. Any appropriate protocolknown in the art may be used for the request; one particularnon-limiting example of a suitable protocol is the well-known RTSPprotocol, described, for example in documents published on the WorldWide Web (WWW) at www.ietf.org/rfc/rfc2326.txt, and in later draftsfound at www.rtsp.org/drafts/. In such a client-server network, theserver typically streams video frame data to the client, and suppliesECMs as necessary. Persons skilled in the art will appreciate that, in ahome network environment, the frame provision system 40 is typicallylocated in a server remote from the television 15 and that otherconventional components (not shown) are provided in order to implementthe network.

In a non-server (“local”) PVR environment, such as that shown in FIG. 1,speed of fast forward is typically set by an internal call within thePVR, so that a specific protocol need not be used.

The frame provision system 40 preferably performs the following tasks:identification of frames to be rendered in order to fulfill the request;and substitution of another frame for at least one of the identifiedframes, in order to reduce a total number of key periods for which aframe is to be rendered. The frame provision system 40 then preferablyprovides a list (not shown in FIG. 1) of frames to be rendered to thetrick modes unit 30. While identification of frames to be rendered andsubstitution of another frame are described throughout the presentspecification as separate tasks or steps, persons skilled in the artwill appreciate that, for purposes of optimized performance, the listmay also be produced in a single step.

Reference is now additionally made to FIG. 2, which is a simplifiedpictorial illustration of scrambled frames comprised in a plurality ofkey periods, useful for understanding the system of FIG. 1; and to FIG.3, which is a simplified pictorial illustration of scrambled framescomprised in a plurality of key periods, showing substitution of framesto be descrambled in order to reduce a number of key periods for which aframe is to be descrambled, useful for understanding the system of FIG.1.

FIG. 2 depicts, in highly simplified form, frames and their associatedkey periods. In FIG. 2 four key periods, numbered 1 through 4, areshown, each key period comprising four scrambled frames 100. It isappreciated that the depiction of key periods and scrambled frames 100in FIG. 1 is highly simplified for purposes of simplicity of depictionand description; in practice, a key period might include, for example, amultiplicity of scrambled frames 100, such as, for example, two hundredscrambled frames 100.

In FIG. 2, a plurality of frames to be descrambled 110 has beenidentified by the frame provision system 40 of FIG. 1. In, for example,a case of a fast forward request, the frames to be descrambled 110represent frames to be rendered in order to fulfill the fast forwardrequest.

In the highly simplified example of FIG. 2, one in every three scrambledframes 100 is to be descrambled. Since each key period in FIG. 2comprises four scrambled frames 100, it will be appreciated that atleast one frame to be descrambled 110 is comprised in each key period.Thus, it would be necessary to produce a key for each key period; it isbelieved that producing a key for each key period might strain theabilities of the system of FIG. 1, as discussed above.

In FIG. 3, some of the frames to be descrambled 110 have been replacedby substituted frames 120, in such a way as to reduce the number of keyperiods for which a frame is to be descrambled. In the highly simplifiedexample of FIG. 3, the frames to be descrambled (110, 120) are comprisedin only two of the four key periods shown. Thus, the necessity toproduce keys has been greatly reduced, and so also has the likelihood ofstraining the abilities of the system of FIG. 1.

It is appreciated that the frames to be descrambled (110, 120) shown inFIG. 3 do not comprise “evenly spaced” frames, as do the frames to bedescrambled 110 of FIG. 2. However, the inventors of the presentinvention believe that, when the frames to be descrambled (110, 120) ofFIG. 3 are rendered, a person viewing the frames is unlikely to noticeobjectionable effects.

It is appreciated that, in actual operation of the system of FIG. 2,selection of frames to show in order to achieve a desired fast forwardspeed may change dynamically based on system load; for instance, if adisk, network, ECM-providing, or other component is fully loaded at agiven time, fewer frames may be shown by jumping further between frameswhich are shown and/or by reducing the need for ECM processing byskipping entire key periods.

Reference is now made to FIG. 4, which is a simplified block diagramillustration of a preferred implementation of a portion of the system ofFIG. 1, comprising the frame provision system 40. The elements of FIG. 4are preferably implemented in any suitable combination of hardware andsoftware. It is appreciated that the elements of FIG. 4 are shown anddescribed functionally; persons skilled in the art will appreciate thatthe elements of FIG. 4 need not be implemented in separate units.

The frame provision system 40 preferably comprises, in addition to theframe substitutor 50, a frame identifier 410 and a key period identifier420.

The frame provision system 40 receives a request 430 as described abovewith reference to FIG. 1. The request is sent to the frame identifier410, which identifies a plurality of frames to be rendered (such as, forexample, the frames to be descrambled 110 of FIG. 2).

A list of the frames to be rendered is provided by the frame identifier410 to the key period identifier 420 and to the frame substitutor 50.The key period identifier 420 preferable identifies a list of keyperiods to which the list of frames belongs, and provides the list ofkey periods to the frame substitutor 50.

The frame substitutor 50 preferably utilizes the list of frames providedby the frame identifier 410 and the list of key periods provided by thekey period identifier 420 to identify one or more frames to besubstituted, as described above with reference to FIG. 1. It isappreciated that, in some cases, in may not be necessary or desirable tosubstitute any frames, in which case it is preferable that no frames aresubstituted by the frame substitutor 50. The frame substitutor thenprovides a frame list 440 for further processing such as, for example,rendering.

Reference is now made to FIG. 5, which is a simplified flowchartillustration of a preferred method of operation of the system of FIG. 1.The method of FIG. 5 preferably comprises the following steps:

A request to perform a trick mode, such as fast forward or fast reverse,is received (step 510). It is appreciated that step 510 is optional, andthat the remaining steps of the method of FIG. 5 may be performedwithout step 510.

A plurality of frames to be descrambled is identified (step 520),preferably as described above with reference to FIGS. 1 and 4. For eachof the frames to be descrambled, a key period to which the frame belongsis identified (step 530).

For at least one of the frames to be descrambled, another frame issubstituted, in order to reduce the total number of key periods to whichthe frames belong (step 540), preferably as described above withreference to FIGS. 1 and 4.

Reference is now made to FIG. 6, which is a simplified flowchartillustration of a preferred implementation of a portion of the method ofFIG. 5; the method of FIG. 6 comprises a preferred implementation ofsteps 520, 530, and 540 of FIG. 5. It is appreciated that the method ofFIG. 6 comprises one particular preferred embodiments; the method ofFIG. 6 is not meant to be limiting.

The method of FIG. 6 preferably comprises the following steps:

A number of key periods to be skipped for each key period to bedescrambled is determined (step 610); for example, let the number of keyperiods to be skipped be denoted KN. As described above, thedetermination of KN is preferably dynamically based on system load; forinstance, if a disk, network, ECM-providing, or other component is fullyloaded at a given time, fewer frames may be shown by skipping entire keyperiods.

A plurality of groups of key periods is determined, each group includingone key period to be descrambled and at least one key period to beskipped (step 615). It will be appreciated that, in general, each suchgroup will include KN key periods to be skipped.

A number N of frames that should be provided, for example, provided forrendering, for each group of key periods is determined (step 620). Apreferred method for determining N is as follows (with “*” representingmultiplication and “/” representing division):

-   -   1. Let M be a number such that, for the given fast forward (or        other trick mode) request being processed, 1 in M frames is to        be rendered.    -   2. Let K be the number of frames in each key period; persons        skilled in the art will appreciate that, in practice, K may be        the approximate number of frames in each key period, since the        number of frames in a key period may differ from one key period        to another key period.    -   3. The inventors of the present invention believe it is        reasonable, in the presently describe preferred method, to        assume that: 2*M<K*(KN+1); equivalently, it is reasonable to        assume that at least two frames are to be descrambled for each        group of key periods.    -   4. N is preferably computed as:        N=(KN+1)*K/M

For each group of key periods in step 620, N frames are chosen to beprovided or rendered; the N frames are chosen from within the one keyperiod to be descrambled for that group of key periods (step 630).

A preferred method for choosing the N frames in the one key period to bedescrambled is as follows:

-   -   1. Let F be the number of the first frame in the key period.    -   2. Let L be the number of the last frame in the key period.    -   3. Choose F and L as 2 of the frames to be descrambled. If N=2,        the preferred method for choosing the N frames terminates.        Otherwise:    -   4. Divide the range F . . . L into N−1 parts and choose frames        “evenly spaced” within the range.

Another way of expressing the preferred method for choosing the N framesin the one key period to be descrambled is as follows:

-   -   The frames to decode are frames numbered:        F+(X−1)*(L−F)/(N−1)    -   where X ranges from 1 to N.

It is appreciated that various features of the invention which are, forclarity, described in the contexts of separate embodiments may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment may also be provided separately or in anysuitable subcombination.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the invention is defined onlyby the claims which follow:

1-21. (canceled)
 22. A method for processing scrambled content, thescrambled content being divided into frames, the method comprising:identifying a first plurality of frames to be descrambled, the firstplurality of frames being associated with a second plurality of keyperiods; choosing a first key period of the second plurality of keyperiods; and identifying all frames of the first plurality of frameswhich are associated with the first key period and substituting, foreach frame identified, a substitute frame not associated with the firstkey period to produce a new first plurality of frames, thereby reducinga total number of key periods associated with keys for descrambling allof the new first plurality of frames.
 23. The method according to claim22 and also comprising: producing one key for each key period of thereduced total number of key periods.
 24. The method according to claim22 and also comprising: producing exactly one key for each key period ofthe reduced total number of key periods.
 25. The method according toclaim 23 and wherein the producing comprises producing from an ECM. 26.The method according to claim 24 and wherein the producing comprisesproducing from an ECM.
 27. The method according to claim 22 and alsocomprising: rendering at least one of the new first plurality of frames.28. The method according to claim 27 and wherein the rendering comprisesrendering all of the new first plurality of frames.
 29. Apparatus forprocessing scrambled content, the scrambled content being divided intoframes, the apparatus comprising: a frame identifier identifying a firstplurality of frames to be descrambled, the first plurality of framesbeing associated with a second plurality of key periods; a key periodidentifier choosing a first key period of the second plurality of keyperiods; and a frame substitutor identifying all frames of the firstplurality of frames which are associated with the first key period andsubstituting, for each frame identified, a substitute frame notassociated with the first key period to produce a new first plurality offrames, thereby reducing a total number of key periods associated withkeys for descrambling all of the new first plurality of frames. 30.Apparatus according to claim 29 and also comprising: key productionapparatus for producing one key for each key period of the reduced totalnumber of key periods.
 31. Apparatus according to claim 30 and whereinthe key production apparatus is operative to produce exactly one key foreach key period of the reduced total number of key periods. 32.Apparatus according to claim 30 and wherein the producing comprisesproducing from an ECM.
 33. Apparatus according to claim 31 and whereinthe producing comprises producing from an ECM.
 34. Apparatus accordingto claim 29 and also comprising: rendering apparatus for rendering atleast one of the new first plurality of frames.
 35. Apparatus accordingto claim 34 and wherein the rendering comprises rendering all of the newfirst plurality of frames.